This invention relates to an improved soda pulping process for delignifying lignocellulosic materials such as wood, whole-tree chips, bagasse, straw, kenaf, reeds, and other plants and crops.
The most commonly used chemical pulping process, kraft (or sulfate) pulping, is versatile with respect to possible raw materials and cooking conditions. Its disadvantages include high capital costs, malodorous gaseous emissions, and a lack of selectivity for delignification at lower yields, whereby some of the cellulosic component of the raw material is degraded, reducing the yield of pulp.
The soda pulping process, though free from the air pollution problems of kraft pulping, usually requires much longer cooking times, and gives low yields of pulp having strength characteristics inferior to kraft pulp.
Holton teaches in a recent publication and patent (Pulp and Paper Canada 78 (10):T218 (1977), U.S. Pat. No. 4,012,280, Mar. 15, 1977) that the addition of a small amount of a cyclic keto compound, such as anthraquinone (AQ), accelerates soda pulping to kraft-like rates and yields. Soda-AQ pulping does not, however, produce pulps equal in strength, especially tear strength, to kraft pulps at comparable yields and kappa numbers (see Table I).
For example, the above-cited publication by Holton for the pulping of a mixture of spruce, balsam and pine shows the kraft control at abnormally low total yield and kappa number values for such an unbleached softwood pulp, making the soda-AQ pulp unrealistically favorable by comparison. Our data for similar pulping of black spruce (Table I) show that relative to normal kraft pulp, at conventional yields and kappa number values, unbleached soda-AQ pulp has much lower viscosity, 6% lower tensile, 22% lower tear, 10% lower burst, and 41% fewer folds. U.S. Pat. No. 4,012,280 teaches that after conventional CEDED bleaching, fully bleached soda-AQ pulp is 37% lower in viscosity, 4% lower in tear, and 5% lower in burst than the bleached kraft control. Again, the unbleached pulp has abnormally low total yield and kappa number values for such a kraft pulp.
TABLE I. __________________________________________________________________________ PHYSICAL PROPERTIES OF UNBLEACHED AND BLEACHED KRAFT AND SODA-AQ PULPS FROM SOFTWOOD.sup.a UNBLEACHED BLEACHED Pulp & Paper Canada U.S. Pat. No. 78(10):T218 (1977) This work 4,012,280 This work Kraft Soda-AQ Kraft Soda-AQ Kraft Soda-AQ Kraft Soda-AQ SBP SBP BS BS SBP SBP BS BS __________________________________________________________________________ Total yield, % 44.2 48.7 48.2 48.6 47.0 48.7 48.8 51.1 Unbleached Kappa number 25.2 30.5 31.2 29.3 28.5 30.2 30.3 30.5 Unbleached viscosity, mPa .multidot. s -- -- 32.4 20.6 28.3 14.8 34.7 21.0 Tensile, km 11.7 12.3 14.9 14.0 11.8 11.8 14.4 14.3 Tear index, mN .multidot. m.sup.2 /g 9.0 9.4 12.0 9.4 10.3 9.9 9.5 8.8 Burst index, kPa .multidot. m.sup.2 /g 9.0 9.9 12.3 11.1 10.3 9.8 12.0 11.2 Bulk, cm.sup.3 /g 1.20 1.30 1.34 1.37 1.30 1.40 1.27 1.31 Elongation, % -- -- 4.0 3.2 3.0 2.7 3.9 3.5 PFI revs 10,700 9,000 9,800 9,400 -- -- 10,100 9,800 Folds, MIT -- -- 4010 2350 -- -- 3990 -- Brightness, % 88.1 88.7 90.0 85.7 __________________________________________________________________________ .sup.a AQ at 0.25% on O.D. wood; all mechanical strength properties at 30 ml CSF. SBP = Spruce, balsam, pine. BS = Black spruce.
Two of us, viz., Kubes and Bolker, reported at the TAPPI Alkaline Pulping Conference preprints, Washington, D.C., November, 1977, that the addition of a relatively large quantity of certain amino compounds (e.g., ethylenediamine (EDA)) to soda liquor resulted in pulping rates equal to or faster than that of kraft pulping, and gave pulps with superior mechanical strength properties, especially tear strength (see Table II). A disadvantage of soda-amine pulping was the high initial concentration of amine (typically at least 10% by weight, based on dry raw material) required to produce the desired effects.
We have now discovered that by adding to an alkaline, i.e., soda-type, pulping liquor very small quantities of both a cyclic keto compound and ethylenediamine or like amino compound, an unexpected synergistic effect is achieved, viz., with small quantities of both it is possible not only to delignify lignocellulosic materials at rates comparable to kraft pulping but also to obtain good yields of pulps having physical strength properties (especially tear strength) which are equal to, or better than those of comparable kraft pulps. For example, if the amino compound is EDA, the synergistic effect is such that only 0.1% by weight thereof on wood in combination with 0.1% by weight on wood of AQ, will give a pulp having 15-20% higher tear than that of soda-AQ pulp. Similarly, the synergistic effect improves the accelerating efficiency of the cyclic keto compound so that its charge may be significantly decreased, e.g., to 0.1% by weight on wood, without affecting the delignification rate.
TABLE II. ______________________________________ PHYSICAL PROPERTIES OF UNBLEACHED SODA, KRAFT, AND SODA-EDA PULPS FROM BLACK SPRUCE.sup.a Soda Soda-EDA Kraft ______________________________________ Total yield, % 43.8 47.3 48.2 Kappa number 31.5 33.4 31.2 Viscosity, mPa .multidot. s 9.4 27.5 32.4 Maximum cooking temperature, .degree.C. 172 166 166 Time to temp., min. 90 90 90 Time at temp., min. 165 100 168 Tensile, km 11.9 11.4 14.2 TEAR INDEX, mN .multidot. m.sup.2 /g 10.2 18.7 11.3 Burst index, kPa .multidot. m.sup.2 /g 8.6 9.6 11.0 Bulk, cm.sup.3 /g 1.44 1.48 1.37 Elongation, % 2.7 4.0 3.8 PFI revs 4,600 11,400 4,900 Folds, MIT 1780 2870 2630 ______________________________________ .sup.a All mechanical strength properties at 500 ml CSF; data from G. J. Kubes and H. I. Bolker, TAPPI Alkaline Pulping Conference preprints, Washington, D.C., November, 1977. .sup.b EDA at 40% on O.D. wood.
The use of these combined accelerators provides a pulp in higher yield at a much faster delignification rate than a similar process without the combined additives. The low doses of additives are economically favorable, chemical recovery of cooking chemicals is simplified, and the environmental pollutants of kraft pulping are decreased or eliminated.
It is a primary object of the invention to provide a soda-type pulping process which gives high yields of cellulosic pulps having physical strength properties comparable to, or better than, those of kraft pulps at equivalent yields. A second object is to delignify the raw material quickly, thus conserving energy and increasing throughput. Another object is to increase pulping rates and yields using smaller amounts of pulping accelerators. A further object is to provide a pulping process in which the discharge of gaseous and aqueous pollutants is decreased or eliminated. Other objects will be apparent to those skilled in the art.